1. Field of the Invention
The present invention relates to a method of forming patterns and an apparatus for carrying out the same whereby fine patterns are formed easily and with precision. More particularly, the invention relates to a method of forming patterns and an apparatus for carrying out the same in conjunction with the manufacture of large scale integrated circuits (LSI's) by use of projection aligner.
2. Description of the Prior Art
Fine patterns of semiconductor integrated circuits and the like are formed by the so-called reduction projection aligner. What the aligner does is to project by exposure, on a reduced scale, circuit patterns drawn on a mask or reticle (both are generically referred to as the reticle) onto a substrate (generally called the wafer) through an exposure lens. The circuit pattern image must be focused on the wafer surface precisely, i.e., with a good resolution. Recently developed reduction projection aligners tend to have greater numerical aperture (NA) than earlier models. The reason for this improvement is to further reduce optically printable line widths (below the submicron order). The novel capability involves reduced practical depths of focus of the projection lens. This means a greater need for more precise alignment of the projection image plane (focal plane) with the wafer surface. That precise alignment needs to be achieved by aligners equipped with a focusing function.
One such aligner with the focusing function is disclosed illustratively in Japanese Patent Laid-Open No. SHO/60-168112. The disclosed apparatus varies the distance between wafer and projection lens in the optical axis direction to obtain the best focal plane through detection. Another aligner with the focusing function is disclosed in Japanese Patent Laid-Open No. SHO/61-290306. This apparatus involves initially forming interference fringes on the wafer by use of two light beams and then varying the distance between wafer and projection lens in the optical axis direction so as to obtain the plane where the diffracted light intensity from the wafer is the highest. Another aligner with the focusing function, disclosed in Japanese Patent Laid-Open No. HEI/1-262624, illuminates fiducial marks at different incident angles using two light beams of different polarization characteristics. The projected images of the fiducial marks are used to scan reticle marks. The resulting position error in the scanning direction allows a focal error to be calculated, whereby correct focusing is achieved. Yet another aligner with the focusing function is disclosed in Japanese Patent Laid-Open No. SHO/60-78454. The apparatus first calculates the accumulated amount of heating in the projection lens arrangement caused by the incident light irradiated thereto. The calculated amount of heating is used to vary the air pressure in the enclosed space within the projection lens arrangement so as to obtain the best focal plane.
The apparatus disclosed in Japanese Patent Laid-Open No. SHO/60-168112 requires shifting the wafer bit by bit in the optical axis direction. In each of the slightly shifted positions, the wafer marks need to be detected and their image contrasts obtained. Although the amount of focal error is accurately detected and the error is compensated with high reliability, it takes time for the apparatus to attain correct focusing. This tends to reduce the throughput of the apparatus. The apparatus disclosed in Japanese Patent Laid-Open No. HEI/1-262624 offers improved levels of throughput because it detects the amount of focal error from the detected marks based on a single scan using two light beams of different incident angles. However, this apparatus cannot secure a sufficiently large difference between the two incident angles because it illuminates the fiducial marks at these angles using P and S waves obtained by passing the light through divided polarizers at the fiber tip. Furthermore, because the P and S waves are detected with two different detectors for comparison, the detectors may develop a detection error that stems from their potentially different detection characteristics. This adversely affects the reliability of the detected results.
The apparatus disclosed in Japanese Patent Laid-Open No. SHO/60-78454 compensates any focal plane error by calculating the amount of focal error based on the accumulated amount of heating in the projection lens arrangement. One disadvantage of this apparatus is that it is difficult to ascertain whether correct focusing is really attained. Minute and unpredictable ambient changes can lead to a focusing error, which cannot be compensated. This has a detrimental effect on the reliability of the apparatus.
As described, most prior art aligners with the automatic focusing function have been deficient in calibrating the operating conditions of the focusing mechanism by detecting precisely and quickly that focal plane of the projection optics which changes over time, and in accurately positioning the wafer surface to the correct focal plane under the calibrated operating conditions.